python: add mypy support and some type hints

`deltachat.const` module now defines `__getattr__` and `__dir__` as
suggested by https://www.python.org/dev/peps/pep-0562/
mypy detects that `__getattr__` is defined and does not show errors
for `DC_*` constants which cannot be detected statically.
mypy is added to `tox.ini`, so type check can be run with `tox -e mypy`.

python/tox.ini

@@ -3,6 +3,7 @@ isolated_build = true
 envlist =
     py3
     lint
+    mypy
     auditwheels
 
 [testenv]
@@ -43,6 +44,15 @@ commands =
     flake8 tests/ examples/
     rst-lint --encoding 'utf-8' README.rst
 
+[testenv:mypy]
+deps =
+    mypy
+    typing
+    types-setuptools
+    types-requests
+commands =
+    mypy --no-incremental src/
+
 [testenv:doc]
 changedir=doc
 deps =